I worked for years at two technology periodicals—one dedicated to PCs and the other to Apple products. For the longest time, the two periodicals had different styles for handling "iWhatever" product names, as well as product names with such intendedly eye-catching typographical treatments as "alllowercase" or "ALLCAPS" or "InterCapPing" or "hOWABOUTHIS" or "Cute*Punc|tuation" or "OMGiHATETHISPRODUCT!!!"
At the Apple-focused periodical, Apple product-naming conventions were sacrosanct: You'd as soon call Steve Jobs a martinet as capitalize the first n in "iPod nano" once the great corporation had decreed the correct treatment to use. This, of course, led to slippage on other fronts. If "iPod nano," why not "Yahoo!" and "c|net" and "E*TRADE" and "nVIDIA" and some weird product name with superscript letters and indecipherable Led Zeplin–like icons?
At the PC-focused periodical, we drew a line in the sand and held our position behind it for years: Initial-cap product names, whether the purveyor did or not; reduce intercaps and all-cap interiors to lowercase, and remove all in-name and end-name punctuation. This yielded "Ipod Nano," "Yahoo," "Cnet," Etrade," and "Nvidia"—and countless letters to the editor from consumers outraged on behalf of the poor, disrespected product names. You'd think that removing the screaming final-word treatment from "Gold Disk ASTOUND!" was a personal affront to every person who owned a copy of the program.
All of this is by way of submitting the thesis that publishers are under no obligation to product vendors—or their intellectual property lawyers—to replicate the exact capitalization and punctuation used in a product name, any more than they are obliged to replicate the typeface and color of a company's logo whenever they mention the company by name. As a publisher, you get to decide how far your periodicals will go in reinforcing the vendors' marketing decisions (because that's what product-name typography decisions are) about eyeball capture and brand recognition and shepherding potential customers into the "buying funnel."
With regard to capitalizing or lowercasing the i in iPhone or the e in eBay when the word happens to begin a sentence, I would advise you to use the same capping and lowercasing that you've chosen to use when the product name comes up in the middle of a sentence or in a stand-alone label. Regardless of how you've decided to style the product name there, it is a proper name, and it doesn't cease to be a proper name—or suddenly become a less adequate proper name—just because it pops up in a headline or at the beginning of a sentence.